Digital optical networks such as synchronous optical network (SONET) and synchronous digital hierarchy (SDH) provide the predominant transmitting and multiplexing standards for high-speed signals used in communications and computer networks today. SONET is the most widely used standard in North America and it is often said that SDH is the international version of SONET. These two standards are compatible with one another and essentially transmit data using the same protocols and architecture. Although terminology between the two standards varies, both the SONET and SDH architectures format data into high-speed frames, each having a standard number of bytes. For simplicity's sake, a brief discussion of the SONET data structure using SONET terminology is provided below.
The basic building block of a SONET digital transmission system is a synchronous transport level one, or STS-1, frame which consists of 9 rows by 90 columns of bytes, for a total of 810 bytes. The frames are transmitted at a rate of 8,000 frames per second (or once every 125 microseconds) to provide a 51.84 Mbps signal rate. The STS-1 frame is transmitted one row at time, from top to bottom, and from left to right within each row. Therefore, the byte in row 1, column 1 is sent first, and the byte in row 9, column 90 is sent last. After the 90th byte is sent at the end of row 1, the next byte sent is the first byte in row 2, the byte in column 1. Because one frame is sent every 125 microseconds, SONET can maintain time-slot synchronization that is required for delivery of PCM voice data (8 bits at 8,000 times per second or 64 kbps). SONET also adheres to frame synchronization time with asynchronous network standards such as DS-1, E-1, and DS-3.
FIG. 1 illustrates the data format for a SONET STS-1 frame 100 having 9 rows by 90 columns of bytes (otherwise referred to as “octets”). The first three columns 102 are allocated for transport overhead (TOH) information which includes section overhead (SOH) and line overhead (LOH) data. As is known in the art, SOH data deals with the transport of an STS frame across the physical medium and controls functions such as framing the SONET data stream, scrambling and error monitoring. The LOH data deals with the reliable transport of the payload between line terminating equipment. The remaining 87 columns of the STS-1 frame consist of 783 octets (9 rows×87 columns) that are allocated for “user” data, otherwise referred to as the “payload.” The structure of the payload is defined by a synchronous payload envelope (SPE) 200 which contains 783 octets of data and is transmitted at 8,000 times per second. The first column 112 of SPE 110 contains additional overhead information, commonly referred to as path overhead (POH) data, as well as the actual user data. The POH data 112 is stored in one “column” or nine bytes of the SPE. The first POH byte indicates the first byte of the SPE. The POH data is used to monitor and manage the transport of network services such as DS1 or DS3, for example, between path terminating equipment (PTE).
Higher rate SONET formats essentially follow the same format of the STS-1 frame. All SONET frames contain exactly nine rows and are transmitted at 8,000 times per second. The only variable is the number of columns, or subcolumns. For example, an STS-3 frame consists of 9 rows and is sent 8,000 times per second; however, an STS-3 frame is not 90 columns wide, but is three times wider. Therefore, the STS-3 frame is 270 columns wide and its corresponding transmission rate is 155.52 Mbps. The STS-3 overhead columns are multiplied by three as well, as are the SPE capacity columns. Typically, the STS-3 frame comprises three STS-1 signals interleaved as alternating columns within the STS-3 frame. Therefore, the first column of the STS-3 frame is the first column of a first STS-1 signal, the second column of the STS-3 frame is the first column of a second STS-1 signal, and so on. Similarly, higher order STS-N formats (e.g., STS-12, STS-48, etc.) have proportionally wider frame formats containing a greater number of interleaved columns and faster bit transmission rates. Today, a STS-192 signal can transmit data at a rate of 9.953 Gbps!
With the ever-increasing bandwidths of digital signals being provided by optical networks, the required speed and power of processors to handle these high-bandwidth data streams has been a constant challenge to processor designers and engineers. One of the primary limitations for handling higher bandwidth data streams is the ability of the processors to handle such enormous quantities of data. Therefore, there is a constant need to provide faster and more powerful processing engines to handle the higher-bandwidth signals capable of being transmitted in digital optical networks today.